Gregory Eberle

Experimental and modelling investigations into the laser ablation with picosecond pulses at second harmonics

Ablation threshold experiments on various materials are carried out using a picosecond laser generating second harmonic radiation in air at atmospheric pressure. Various materials are investigated which vary according to their different electronic band gap structure and include: silicon, fine grain polycrystalline diamond, copper, steel and tungsten carbide. Through the use of scanning electron microscopy and 3D confocal microscopy, the crater depth and diameter are determined and a correlation is found. The ablation thresholds are given for the aforementioned materials and compared with recent literature results. Picosecond laser-material interactions are modelled using the two-temperature model, simulated and compared with experimental results for metallic materials. An extension of the two-temperature model to semiconducting and insulating materials is discussed. This alternative model uses multiple rate equations to describe the transient free electron density. Additionally, a set of coupled ordinary differential equations describes the processes of multiphoton excitation, inverse bremsstrahlung, and collisional excitation. The resulting electron density distribution can be used as an input for an electron density dependent twotemperature model. This multiple rate equation model is a generic and fast model, which provides important information like ablation threshold, ablation depth and optical properties.

Focal length stabilization of a tunable lens integrated focus shifting unit

A focus shifting unit integrated with a tunable lens allows for rapid response times, high accuracy, small footprint and simple controllability without the need for any translational mechanics. The focus shifting unit is designed for laser material microprocessing applications where tolerances of a few micrometers are required. However, the optical fluid inside the tunable lens can be severely altered by long term thermal influences from the environment and high powered laser beams. Utilizing the working principles of a cylinder lens, a discrete proportional-integral-derivative controller with an anti-reset windup is simulated and designed for offline regulation of the focal length of the tunable lens. This allows for integration into a three-dimensional scanhead system to reliably deflect the focused laser spot at the workpiece level over long periods of time, i.e. > 8 hours. Deviation of the focal length of the tunable lens is identified by the cylinder lens through ellipticity of a probe laser beam. The focal length is subsequently corrected by altering the input current into the tunable lens by means of the control system which is based on numerical methods. The thermal behavior of the tunable lens, system identification and synthesis of the controller, design of the focus shifting optical system and validation of the controller are studied.